The invention relates to a write-once nonvolatile phase change memory array. In preferred embodiments, such an array can be a monolithic three dimensional memory array of stacked memory levels.
Phase-change materials such as chalcogenides have been used in nonvolatile memories. Such materials can exist in one of two or more stable states, for example a high-resistance and a low-resistance state. In chalcogenides, the high-resistance state corresponds to an amorphous state, while the low-resistance state corresponds to a more ordered crystalline state. The conversion between states is generally achieved thermally.
Accomplishing the phase change from a crystalline state to an amorphous state is achieved by applying a sharp pulse to the phase change material, heating it to high temperature, and allowing rapid cooling. The opposite conversion, from the crystalline state to the amorphous state, requires more careful control, heating the phase-change material to a moderate temperature and cooling it slowly.
This crystalline-to-amorphous conversion becomes increasingly difficult to control as the array increases in size. A need exists, therefore, to form a nonvolatile phase-change memory array having a large array size.